Entrance Exams
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The evolutionary trend shows progressive increase in sporophytic dominance, development of xylem-phloem in pteridophytes, transition to seed production in gymnosperms, and finally evolution of flowers in angiosperms.
The archegonium is a flask-shaped female reproductive organ that protects the egg cell and provides a passage (neck canal) for sperm entry, requiring moisture for fertilization.
These characteristics are typical of pteridophytes/ferns: independent sporophyte and gametophyte, sporangia on fern fronds, and reproduction through spores without seeds.
The integument develops from the chalaza end of the nucellus and grows upward to cover the nucellus, leaving only the micropyle opening for pollen entry.
Flowers attract pollinators through various mechanisms (color, scent, nectar) and provide protection to reproductive organs, making sexual reproduction more efficient compared to gymnosperms.
The integration of female gametophyte (nucellus tissue) within the ovule in gymnosperms shows reduction of gametophytic independence and represents a major evolutionary transition from pteridophytes to seed plants.
Heterospory in Selaginella produces microspores (male) and megaspores (female), representing an evolutionary step toward seeds, and megaspores can produce female gametophytes that are partially protected, reducing water dependence for fertilization.
In angiosperms, the diploid sporophyte produces haploid spores through meiosis, which develop into haploid gametophytes that produce haploid gametes.
The cushion-like growth form in mosses helps in water retention, provides mechanical support without vascular tissue, and increases surface area for photosynthesis and water absorption.
The prothallus (gametophyte) in pteridophytes is homologous to the gametophytic generation in bryophytes as both are haploid structures producing gametes.